Abstract

<p>An area at the center of the human fovea, subtending a visual angle of only 7–8 min and hence hardly larger than the fixation area, is blue-blind in the sense of almost or entirely lacking blue-sensitive cones. This is a matter of foveal topography, not size of field, for in fields of this size elsewhere in the fovea or in the parafovea, blue-sensitive cones are well represented. The blue-cone system falls in sensitivity from the border of the photopic zone—the functionally all-cone area—to a minimum, usually to extinction, at its center. Other features of foveal topography oppose this trend: the density of cones rises and the macular pigmentation thins out toward the center of the fovea. Also the red- and green-cone systems display the opposite gradient; their sensitivities decline regularly from the center toward the borders of the fovea and beyond.</p><p>Tritanopia, though the rarest form of congenital color-blindness, is thus a regular feature of the center of the normal fovea. The existence of two neutral points in this condition, in the yellow and violet, has its basis in the observation that the luminosity curves of the red- and green-sensitive cones, drawn so as to cross in the yellow, cross again or fuse in the violet region. It is suggested that the blue-blindness of the fixation area is a final step in the general withdrawal of image vision from the short wavelengths of the spectrum, for which the chromatic aberration of the eye is greatest. The blue-blindness of the fixation area, taken together with the red-green blindness of more-or-less concentric zones of the near periphery, and the total colorblindness of the far periphery, raises the possibility that various zones of the normal retina display all the major forms of colorblindness. Trichromic vision is normal only in the broad, central annulus of the retina, which alone is ordinarily tested. Some instances of defective color vision may be similarly localized. The problems of both normal and defective color vision involve not only the presence or absence of certain visual pigments and types of cone, but their spatial distributions on the retinal surface, and their neural connections.</p>

© 1967 Optical Society of America

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  1. A. König and E. Köttgen, Sitzber. Akad. Wiss. Berlin, 1894, p. 577, A. König, Gesammelte Abhandlungen zur Physiologischen Optik (J. A. Barth, Leipzig, 1903), p. 338.
  2. I am greatly indebted to Professor Russell Carpenter of Tufts University for permission to use this photomicrograph.
  3. S. L. Polyak, The Retina (Univ. Chicago Press, 1941), pp. 197–199, 447–449. 1289
  4. A. Rochon-Duvigneaud, Les Yeux et la Vision des Vertébrés (Masson et Cie., Paris, 1943), pp. 16–27.
  5. G. Wald, (a) Science 101, 653 (1945). (b) Doc. Ophthalmol. 3, 94 (1949).
  6. Y. Le Grand, Oplique Physiologique, Vol. 3: L'Espace Visuel (Editors Revue d'Optique Paris, 1956), pp. 175–177.
  7. A. König, Sitzber. Akad. Wiss. Berlin, 718 (1897); also in A. König, Gesammelte Abhandlungen (J. A. Barth, Leipzig, 1903), p. 396.
  8. E. N. Willmer, Nature 153, 774 (1944).
  9. E. N. Willmer, J. Theoret. Biol. 1, 141 (1962).
  10. Note the comment by G. L. Walls and R. W. Matthews, New Means of Studying Color Blindness and Normal Foveal Color Vision [Univ. Calif. (Berkeley) Publ. Psychol. 7, No. 1, 158 (1952)]: "The central tetartanopic spot demonstrated in the normal fovea by König, Willmer, Wright, and others probably coincides with … the rod-free area."
  11. E. N. Willmer and W. D. Wright, Nature 156, 119 (1945).
  12. W. S. Stiles (a) Proc. Roy. Soc. (London) B127, 64 (1939). (b) Ned. Tydschr. Natuurk. 15, 125 (1949). (c) Proc. Natl. Acad. Sci. (U. S.) 45, 100 (1959).
  13. E. Auerbach and G. Wald (a) Science 120, 401 (1954). (b) Am. J. Ophthalmol. 39, No. 2, 11, 24 (1955).
  14. W. B. Marks, W. H. Dobelle, and F. F. MacNichol, Jr., Science 143, 1181 (1964).
  15. P. K. Brown and G. Wald, Science 144, 45 (1964).
  16. G. Wald, Science 145, 1007 (1964).
  17. G. Wald, Proc. Natl. Acad. Sci. (U. S.) 55, 1347 (1966).
  18. In a later discussion of the blue-blindness of the "central fovea," Wright re-defined the latter, specifically in this connection, as the central area 20–30 min in subtense [W. D. Wright, Researches on Normnal and Defective Colour Vision (C. V. Mosby, St. Louis, 1947), p. 338]. Through an apparent misunderstanding of Wright's remarks at this point, le Grand ascribed these dimensions also to König's experiments, though I can find no indication of this in König's papers [Y. le Grand, Light, Colour, and Vision (Chapman and Hall, London, 1957), pp. 209, 336].
  19. E. N. Willmer, J. Physiol. (London) 110, 377 (1949); cf. pp. 378–380.
  20. E. N. Willmer, Retinal Structure and Colour Vision (Cambridge Univ. Press 1946), plate facing p. 144.
  21. Also to some degree the duration of stimulus: cf.D. O. Weitzman and J. A. S. Kinney, J. Opt. Soc. Am. 57, 665 (1967).
  22. J. E. Purkinje, Physiological Examtination of the Organ of Vision and the Skin (Univ. Bratislava, Breslau, 1823). Translated in H. J. John, Jan Evangelista Purkyne (Am. Phil. Soc., Philadelphia, 1959), p. 54.
  23. H. Aubert, Physiologie der Netzhaut (E. Morgenstern, Breslau, 1865), pp. 108–124.
  24. H. Hartridge (a) Nature 155, 391 (1945). (b) Nature 155, 657 (1945). (c) Phil Trans. Roy. Soc. (London) B232, 519 (1947).
  25. L. C. Thomson and W. D. Wright, J. Physiol. (London) 105, 316 (1947).
  26. The specifications of the colored background fields are as follows. Field diameter 3.5°, with the test field close to its center. (1) Yellow background, to isolate the blue-sensitive pigment: white field brightness 5060 millilamberts and color temperature 2100°K, passed through Corning filter 3482 plus Jena heat filter KG 1. (2) Purple background to isolate the green-sensitive pigment: white field brightness 20 650 millilamberts and color temperature 2400°K, passed through Wratten filter 35. (3) Blue background to isolate the red-sensitive pigment: white field brightness 16 000 millilamberts and color temperature 2300°K, passed through Wratten filter 47 plus Jena BG18.
  27. F. H. C. Pitt, Proc. Roy. Soc. (London) B132, 101 (1944).
  28. W. D. Wright, J. Opt. Soc. Am. 42, 509 (1952).
  29. G. Østerberg, Acta Ophthalmol. 13, Suppl. 6 (1935). It should be remembered that this work, unique and beautiful as it is, and grateful as we are for it, Nvas pieced out with fragments of a single human retina, and further studies of this kind may be expected to reveal considerable variation from Østerberg's counts.
  30. P. K. Brown and G. Wald, Nature 200, 37 (1963); also unpublished observations.
  31. M. Schultze, Zur Anatomnie und Physiologie der Retina (Max Cohen, Bonn, 1866), Section I; see especially Plate 6, Fig. 1. This monograph appears as a special issue of Arch. Mikr. Anat. 2 (1866).
  32. Y. Le Grand, Light, Colour and Vision (John Wiley & Sons, Inc., N. Y., 1957), pp. 241–244.
  33. G. S. Brindley (a) J. Physiol. (London) 122, 332 (1953). (b) J. Physiol. 124, 400 (1954). (c) with J. J. DuCroz and W. A. H. Rushton, J. Physiol. 183, 497 (1966). (d) Physiology of the Retina and Visual Pathway (Edward Arnold, London, 1960), pp. 235–237.
  34. H. Kalmus, Ann. Human Genetics 20, 39 (1955).
  35. M. von Vintschgau, Pflüger's Arch. Ges. Physiol. 57, 191 (1894).
  36. E. Hering, Pflüger's Arch. Ges. Physiol. 57, 308 (1894).
  37. A. König and C. Dieterici, Z. Psych. Physiol. Sinnesorg. 4, 241 (1892). In A. König, Gesammelte Abhandlungen (A. Barth, Leipzig, 1903), p. 298.
  38. I. Newton, Opticks, Fourth Edition (William Innys, London, 1730). Reprint edition (J. B. Cohen, Ed.) (Dover Publications, Inc., New York, 1952): Book I, Part 2, Prop. 8, p. 165.
  39. G. Wald and D. R. Griffin, J. Opt. Soc. Am. 37, 321 (1947).
  40. G. Wald, Sci. Am. 183, 32 (Aug. 1950).
  41. This term is used here to include both the protanopic and deuteranopic states, in default of information that would permit choice. In such red-green blind areas when fully developed, it reported that reds and greens are confused, and that the only color sensations that persist are blue and yellow.
  42. A Fick, in L. Hermann (Ed.), Jlandbuch der Physiologie (F. C. W. Vogel, Leipzig, 1879), Vol. 3, part I, p. 206.
  43. J. D. Moreland and A. Cruz, Optica Acta 6, 117 (1959). These authors find "strong dichromatic tendencies" at 25–30 deg and monochromacy at 40–50 deg from the fixation point.
  44. W. Nagel, Z. Sinnesphysiol. 44, 5 (1910).
  45. W. Jaeger and K. Kroker, Klin. Monatsbl. Augenheilk. 121, 445 (1952).

von Vintschgau, M.

M. von Vintschgau, Pflüger's Arch. Ges. Physiol. 57, 191 (1894).

Aubert, H.

H. Aubert, Physiologie der Netzhaut (E. Morgenstern, Breslau, 1865), pp. 108–124.

Auerbach, E.

E. Auerbach and G. Wald (a) Science 120, 401 (1954). (b) Am. J. Ophthalmol. 39, No. 2, 11, 24 (1955).

Brindley, G. S.

G. S. Brindley (a) J. Physiol. (London) 122, 332 (1953). (b) J. Physiol. 124, 400 (1954). (c) with J. J. DuCroz and W. A. H. Rushton, J. Physiol. 183, 497 (1966). (d) Physiology of the Retina and Visual Pathway (Edward Arnold, London, 1960), pp. 235–237.

Brown, P. K.

P. K. Brown and G. Wald, Nature 200, 37 (1963); also unpublished observations.

P. K. Brown and G. Wald, Science 144, 45 (1964).

Carpenter, Russell

I am greatly indebted to Professor Russell Carpenter of Tufts University for permission to use this photomicrograph.

Cruz, A.

J. D. Moreland and A. Cruz, Optica Acta 6, 117 (1959). These authors find "strong dichromatic tendencies" at 25–30 deg and monochromacy at 40–50 deg from the fixation point.

Dieterici, C.

A. König and C. Dieterici, Z. Psych. Physiol. Sinnesorg. 4, 241 (1892). In A. König, Gesammelte Abhandlungen (A. Barth, Leipzig, 1903), p. 298.

Dobelle, W. H.

W. B. Marks, W. H. Dobelle, and F. F. MacNichol, Jr., Science 143, 1181 (1964).

Grand, Y. Le

Y. Le Grand, Oplique Physiologique, Vol. 3: L'Espace Visuel (Editors Revue d'Optique Paris, 1956), pp. 175–177.

Y. Le Grand, Light, Colour and Vision (John Wiley & Sons, Inc., N. Y., 1957), pp. 241–244.

Griffin, D. R.

G. Wald and D. R. Griffin, J. Opt. Soc. Am. 37, 321 (1947).

Hartridge, H.

H. Hartridge (a) Nature 155, 391 (1945). (b) Nature 155, 657 (1945). (c) Phil Trans. Roy. Soc. (London) B232, 519 (1947).

Hering, E.

E. Hering, Pflüger's Arch. Ges. Physiol. 57, 308 (1894).

Jaeger, W.

W. Jaeger and K. Kroker, Klin. Monatsbl. Augenheilk. 121, 445 (1952).

Kalmus, H.

H. Kalmus, Ann. Human Genetics 20, 39 (1955).

Kinney, J. A. S.

Also to some degree the duration of stimulus: cf.D. O. Weitzman and J. A. S. Kinney, J. Opt. Soc. Am. 57, 665 (1967).

König, A.

A. König, Sitzber. Akad. Wiss. Berlin, 718 (1897); also in A. König, Gesammelte Abhandlungen (J. A. Barth, Leipzig, 1903), p. 396.

A. König and E. Köttgen, Sitzber. Akad. Wiss. Berlin, 1894, p. 577, A. König, Gesammelte Abhandlungen zur Physiologischen Optik (J. A. Barth, Leipzig, 1903), p. 338.

A. König and C. Dieterici, Z. Psych. Physiol. Sinnesorg. 4, 241 (1892). In A. König, Gesammelte Abhandlungen (A. Barth, Leipzig, 1903), p. 298.

Köttgen, E.

A. König and E. Köttgen, Sitzber. Akad. Wiss. Berlin, 1894, p. 577, A. König, Gesammelte Abhandlungen zur Physiologischen Optik (J. A. Barth, Leipzig, 1903), p. 338.

Kroker, K.

W. Jaeger and K. Kroker, Klin. Monatsbl. Augenheilk. 121, 445 (1952).

MacNichol, Jr., F. F.

W. B. Marks, W. H. Dobelle, and F. F. MacNichol, Jr., Science 143, 1181 (1964).

Marks, W. B.

W. B. Marks, W. H. Dobelle, and F. F. MacNichol, Jr., Science 143, 1181 (1964).

Matthews, R. W.

Note the comment by G. L. Walls and R. W. Matthews, New Means of Studying Color Blindness and Normal Foveal Color Vision [Univ. Calif. (Berkeley) Publ. Psychol. 7, No. 1, 158 (1952)]: "The central tetartanopic spot demonstrated in the normal fovea by König, Willmer, Wright, and others probably coincides with … the rod-free area."

Moreland, J. D.

J. D. Moreland and A. Cruz, Optica Acta 6, 117 (1959). These authors find "strong dichromatic tendencies" at 25–30 deg and monochromacy at 40–50 deg from the fixation point.

Nagel, W.

W. Nagel, Z. Sinnesphysiol. 44, 5 (1910).

Newton, I.

I. Newton, Opticks, Fourth Edition (William Innys, London, 1730). Reprint edition (J. B. Cohen, Ed.) (Dover Publications, Inc., New York, 1952): Book I, Part 2, Prop. 8, p. 165.

Østerberg, G.

G. Østerberg, Acta Ophthalmol. 13, Suppl. 6 (1935). It should be remembered that this work, unique and beautiful as it is, and grateful as we are for it, Nvas pieced out with fragments of a single human retina, and further studies of this kind may be expected to reveal considerable variation from Østerberg's counts.

Pitt, F. H. C.

F. H. C. Pitt, Proc. Roy. Soc. (London) B132, 101 (1944).

Polyak, S. L.

S. L. Polyak, The Retina (Univ. Chicago Press, 1941), pp. 197–199, 447–449. 1289

Purkinje, J. E.

J. E. Purkinje, Physiological Examtination of the Organ of Vision and the Skin (Univ. Bratislava, Breslau, 1823). Translated in H. J. John, Jan Evangelista Purkyne (Am. Phil. Soc., Philadelphia, 1959), p. 54.

Rochon-Duvigneaud, A.

A. Rochon-Duvigneaud, Les Yeux et la Vision des Vertébrés (Masson et Cie., Paris, 1943), pp. 16–27.

Schultze, M.

M. Schultze, Zur Anatomnie und Physiologie der Retina (Max Cohen, Bonn, 1866), Section I; see especially Plate 6, Fig. 1. This monograph appears as a special issue of Arch. Mikr. Anat. 2 (1866).

Stiles, W. S.

W. S. Stiles (a) Proc. Roy. Soc. (London) B127, 64 (1939). (b) Ned. Tydschr. Natuurk. 15, 125 (1949). (c) Proc. Natl. Acad. Sci. (U. S.) 45, 100 (1959).

Thomson, L. C.

L. C. Thomson and W. D. Wright, J. Physiol. (London) 105, 316 (1947).

Wald, G.

P. K. Brown and G. Wald, Science 144, 45 (1964).

G. Wald, Science 145, 1007 (1964).

G. Wald, Proc. Natl. Acad. Sci. (U. S.) 55, 1347 (1966).

E. Auerbach and G. Wald (a) Science 120, 401 (1954). (b) Am. J. Ophthalmol. 39, No. 2, 11, 24 (1955).

G. Wald, (a) Science 101, 653 (1945). (b) Doc. Ophthalmol. 3, 94 (1949).

P. K. Brown and G. Wald, Nature 200, 37 (1963); also unpublished observations.

G. Wald and D. R. Griffin, J. Opt. Soc. Am. 37, 321 (1947).

G. Wald, Sci. Am. 183, 32 (Aug. 1950).

Walls, G. L.

Note the comment by G. L. Walls and R. W. Matthews, New Means of Studying Color Blindness and Normal Foveal Color Vision [Univ. Calif. (Berkeley) Publ. Psychol. 7, No. 1, 158 (1952)]: "The central tetartanopic spot demonstrated in the normal fovea by König, Willmer, Wright, and others probably coincides with … the rod-free area."

Weitzman, D. O.

Also to some degree the duration of stimulus: cf.D. O. Weitzman and J. A. S. Kinney, J. Opt. Soc. Am. 57, 665 (1967).

Willmer, E. N.

E. N. Willmer, J. Physiol. (London) 110, 377 (1949); cf. pp. 378–380.

E. N. Willmer, Retinal Structure and Colour Vision (Cambridge Univ. Press 1946), plate facing p. 144.

E. N. Willmer and W. D. Wright, Nature 156, 119 (1945).

E. N. Willmer, Nature 153, 774 (1944).

E. N. Willmer, J. Theoret. Biol. 1, 141 (1962).

Wright, W. D.

E. N. Willmer and W. D. Wright, Nature 156, 119 (1945).

L. C. Thomson and W. D. Wright, J. Physiol. (London) 105, 316 (1947).

W. D. Wright, J. Opt. Soc. Am. 42, 509 (1952).

Other (45)

A. König and E. Köttgen, Sitzber. Akad. Wiss. Berlin, 1894, p. 577, A. König, Gesammelte Abhandlungen zur Physiologischen Optik (J. A. Barth, Leipzig, 1903), p. 338.

I am greatly indebted to Professor Russell Carpenter of Tufts University for permission to use this photomicrograph.

S. L. Polyak, The Retina (Univ. Chicago Press, 1941), pp. 197–199, 447–449. 1289

A. Rochon-Duvigneaud, Les Yeux et la Vision des Vertébrés (Masson et Cie., Paris, 1943), pp. 16–27.

G. Wald, (a) Science 101, 653 (1945). (b) Doc. Ophthalmol. 3, 94 (1949).

Y. Le Grand, Oplique Physiologique, Vol. 3: L'Espace Visuel (Editors Revue d'Optique Paris, 1956), pp. 175–177.

A. König, Sitzber. Akad. Wiss. Berlin, 718 (1897); also in A. König, Gesammelte Abhandlungen (J. A. Barth, Leipzig, 1903), p. 396.

E. N. Willmer, Nature 153, 774 (1944).

E. N. Willmer, J. Theoret. Biol. 1, 141 (1962).

Note the comment by G. L. Walls and R. W. Matthews, New Means of Studying Color Blindness and Normal Foveal Color Vision [Univ. Calif. (Berkeley) Publ. Psychol. 7, No. 1, 158 (1952)]: "The central tetartanopic spot demonstrated in the normal fovea by König, Willmer, Wright, and others probably coincides with … the rod-free area."

E. N. Willmer and W. D. Wright, Nature 156, 119 (1945).

W. S. Stiles (a) Proc. Roy. Soc. (London) B127, 64 (1939). (b) Ned. Tydschr. Natuurk. 15, 125 (1949). (c) Proc. Natl. Acad. Sci. (U. S.) 45, 100 (1959).

E. Auerbach and G. Wald (a) Science 120, 401 (1954). (b) Am. J. Ophthalmol. 39, No. 2, 11, 24 (1955).

W. B. Marks, W. H. Dobelle, and F. F. MacNichol, Jr., Science 143, 1181 (1964).

P. K. Brown and G. Wald, Science 144, 45 (1964).

G. Wald, Science 145, 1007 (1964).

G. Wald, Proc. Natl. Acad. Sci. (U. S.) 55, 1347 (1966).

In a later discussion of the blue-blindness of the "central fovea," Wright re-defined the latter, specifically in this connection, as the central area 20–30 min in subtense [W. D. Wright, Researches on Normnal and Defective Colour Vision (C. V. Mosby, St. Louis, 1947), p. 338]. Through an apparent misunderstanding of Wright's remarks at this point, le Grand ascribed these dimensions also to König's experiments, though I can find no indication of this in König's papers [Y. le Grand, Light, Colour, and Vision (Chapman and Hall, London, 1957), pp. 209, 336].

E. N. Willmer, J. Physiol. (London) 110, 377 (1949); cf. pp. 378–380.

E. N. Willmer, Retinal Structure and Colour Vision (Cambridge Univ. Press 1946), plate facing p. 144.

Also to some degree the duration of stimulus: cf.D. O. Weitzman and J. A. S. Kinney, J. Opt. Soc. Am. 57, 665 (1967).

J. E. Purkinje, Physiological Examtination of the Organ of Vision and the Skin (Univ. Bratislava, Breslau, 1823). Translated in H. J. John, Jan Evangelista Purkyne (Am. Phil. Soc., Philadelphia, 1959), p. 54.

H. Aubert, Physiologie der Netzhaut (E. Morgenstern, Breslau, 1865), pp. 108–124.

H. Hartridge (a) Nature 155, 391 (1945). (b) Nature 155, 657 (1945). (c) Phil Trans. Roy. Soc. (London) B232, 519 (1947).

L. C. Thomson and W. D. Wright, J. Physiol. (London) 105, 316 (1947).

The specifications of the colored background fields are as follows. Field diameter 3.5°, with the test field close to its center. (1) Yellow background, to isolate the blue-sensitive pigment: white field brightness 5060 millilamberts and color temperature 2100°K, passed through Corning filter 3482 plus Jena heat filter KG 1. (2) Purple background to isolate the green-sensitive pigment: white field brightness 20 650 millilamberts and color temperature 2400°K, passed through Wratten filter 35. (3) Blue background to isolate the red-sensitive pigment: white field brightness 16 000 millilamberts and color temperature 2300°K, passed through Wratten filter 47 plus Jena BG18.

F. H. C. Pitt, Proc. Roy. Soc. (London) B132, 101 (1944).

W. D. Wright, J. Opt. Soc. Am. 42, 509 (1952).

G. Østerberg, Acta Ophthalmol. 13, Suppl. 6 (1935). It should be remembered that this work, unique and beautiful as it is, and grateful as we are for it, Nvas pieced out with fragments of a single human retina, and further studies of this kind may be expected to reveal considerable variation from Østerberg's counts.

P. K. Brown and G. Wald, Nature 200, 37 (1963); also unpublished observations.

M. Schultze, Zur Anatomnie und Physiologie der Retina (Max Cohen, Bonn, 1866), Section I; see especially Plate 6, Fig. 1. This monograph appears as a special issue of Arch. Mikr. Anat. 2 (1866).

Y. Le Grand, Light, Colour and Vision (John Wiley & Sons, Inc., N. Y., 1957), pp. 241–244.

G. S. Brindley (a) J. Physiol. (London) 122, 332 (1953). (b) J. Physiol. 124, 400 (1954). (c) with J. J. DuCroz and W. A. H. Rushton, J. Physiol. 183, 497 (1966). (d) Physiology of the Retina and Visual Pathway (Edward Arnold, London, 1960), pp. 235–237.

H. Kalmus, Ann. Human Genetics 20, 39 (1955).

M. von Vintschgau, Pflüger's Arch. Ges. Physiol. 57, 191 (1894).

E. Hering, Pflüger's Arch. Ges. Physiol. 57, 308 (1894).

A. König and C. Dieterici, Z. Psych. Physiol. Sinnesorg. 4, 241 (1892). In A. König, Gesammelte Abhandlungen (A. Barth, Leipzig, 1903), p. 298.

I. Newton, Opticks, Fourth Edition (William Innys, London, 1730). Reprint edition (J. B. Cohen, Ed.) (Dover Publications, Inc., New York, 1952): Book I, Part 2, Prop. 8, p. 165.

G. Wald and D. R. Griffin, J. Opt. Soc. Am. 37, 321 (1947).

G. Wald, Sci. Am. 183, 32 (Aug. 1950).

This term is used here to include both the protanopic and deuteranopic states, in default of information that would permit choice. In such red-green blind areas when fully developed, it reported that reds and greens are confused, and that the only color sensations that persist are blue and yellow.

A Fick, in L. Hermann (Ed.), Jlandbuch der Physiologie (F. C. W. Vogel, Leipzig, 1879), Vol. 3, part I, p. 206.

J. D. Moreland and A. Cruz, Optica Acta 6, 117 (1959). These authors find "strong dichromatic tendencies" at 25–30 deg and monochromacy at 40–50 deg from the fixation point.

W. Nagel, Z. Sinnesphysiol. 44, 5 (1910).

W. Jaeger and K. Kroker, Klin. Monatsbl. Augenheilk. 121, 445 (1952).

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